Method for selectively plugging subterranean formations with polysulfides

ABSTRACT

The relatively highly permeable zones of a subterranean oil containing formation having a neutral to alkaline pH are selectively plugged by initially injecting an aqueous acid solution into the relatively highly permeable zones that lowers the pH of these zones to about 6 or less. Thereafter, an aqueous polysulfide solution is injected into the formation that preferentially enters the relatively highly permeable zones and allowed to disproportionate under the pH conditions within the relatively highly permeable zones to form a precipitate of elemental sulfur in situ which plugs the relatively highly permeable zones in the formation. Thereafter, improved sweep efficiency is realized in displacing oil from the lesser permeable zones. If the natural pH of the relatively highly permeable zones in the formation is about 6 or less, the initial treatment with the aqueous acid solution is unnecessary.

FIELD OF THE INVENTION

This invention relates to a method for improving the vertical and/orhorizontal sweep efficiency of a subterranean oil-containing formationhaving a neutral to alkaline pH by selectively plugging the relativelyhighly permeable zones in the formation by injecting an aqueous acidsolution into the highly permeable zones to lower the pH to about 6 orless and then injecting an aqueous solution of polysulfides thatdisproportionates under the pH conditions within the relatively highlypermeable zones to form a precipitate of elemental sulfur in situ whichplugs the relatively highly permeable zones.

BACKGROUND OF THE INVENTION

A variety of supplemental recovery techniques have been employed inorder to increase the recovery of viscous oil from subterranean viscousoil containing formations. These techniques include thermal recoverymethods, waterflooding and miscible flooding.

Heterogeneous hydrocarbon containing subterranean formations, i.e.,formations having relatively high permeability zones and relativelylesser permeability zones, are difficult to efficiently flood bysecondary and/or tertiary oil recovery processes because fluidspreferentially migrate into the highly permeable zones in thesubterranean formations. Migration described above is undesirable wheninjecting treatment fluids into oil-containing formations for therecovery oil since the treatment fluids channel through the highlypermeable zones bypassing the less permeable zones. The result is poorconformance and flow profiles of the treatment fluid in the formation.The hydrocarbons residing in the less permeable zones are not producedand the overall yield of hydrocarbons from the formation is reduced.

To increase the efficiency of formation flooding processes, the highlypermeable zones in a subterranean formations are plugged or partiallyplugged to prevent or reduce migration of treatment fluids into them andto divert treatment fluids into adjacent, less permeable zones. Ininjection profile control projects, polymeric materials have been usedin liquid slurries or suspensions to effectively enter and plug orpartially plug the highly permeable and/or fractured zones of theformation. Fluids injected after such a treatment therefore move intounswept areas or zones of the reservoir which results in increased oilrecovery.

Particularly attractive plugging agents include high molecular weightpolymers such as polyacrylamides, polysaccharides and/or cellulosicpolymers with multivalent metal ions and crosslinked products thereof.Many deep, hot reservoirs in the North Sea and Middle East are too hotfor using conventional organic polymers, such as polyacrylamides andpolysaccharides. Polysilicates and modified polyacrylamides may be usedat moderate temperatures, but due to their fast gelling characteristicsonly limited injection is feasible. In addition, these polymers areaffected by salinity and temperature extremes and are eventuallyrendered ineffective by chemical and thermal degradation.

U.S. Pat. No. 3,244,230 discloses a method for reducing the permeabilityof a deep permeable subterranean formation penetrated by a well byintroducing into the formation through the well an aqueous solution ofsodium silicate and urea. The aqueous solution of the sodium silicateand the urea react under the temperature conditions within thesubterranean formation to form a hydrogel of silica that reduces thepermeability of the formation.

U.S. Pat. No. 3,438,439 discloses a method of plugging formations havinga sour gas cap containing hydrogen sulfide by forming elemental sulfurat the location in the formation where the plug or pancake is desired.An aqueous solution of sulfur dioxide is injected at the gas-oilinterface which reacts with the hydrogen sulfide to form a layer of freesulfur.

U.S. Pat. No. 3,376,931 discloses a method for removing elemental sulfurfrom sour reservoir containing hydrogen sulfide gas and free sulfurusing aqueous solutions of ammonium sulfide that dissolve the freesulfur. Removing the sulfur prevents sulfur precipitates from reducingthe permeability of the formation, particularly adjacent the productionwell.

In my copending application Ser. No. 105,436, filed on Oct. 7, 1987,there is disclosed a method for improving the vertical and/or horizontalsweep efficiency of a deep subterranean, oil-containing formation havinga temperature in excess of 250° F. by selectively plugging therelatively highly permeable zones in the formation by injecting anaqueous solution of polysulfides into the formation which preferentiallyenters the relatively highly permeable zones and undergoesdisproportionation under the temperature conditions within therelatively highly permeable zones to form a precipitate of elementalsulfur in situ which plugs the relatively highly permeable zones.

SUMMARY OF THE INVENTION

The present invention provides a method for improving the verticaland/or horizontal sweep efficiency of a subterranean oil-containingformation having a neutral to alkaline pH and having relatively highlypermeable zones and relatively low permeability zones, the formationpenetrated by at least one injection well and one spaced apartproduction well in fluid communication with the formation, comprisinginjecting an aqueous acid solution, preferably hydrochloric acid, thatpreferentially enters the relatively highly permeable zones to lower thepH of the relatively highly permeable zones to a pH of about 6 or less.Thereafter, an aqueous polysulfide solution, preferably ammoniumpolysulfide or ethylamine polysulfide, is injected into the formationthat preferentially enters the relatively highly permeable zones andallowed to disproportionate under the pH conditions within therelatively highly permeable zones to form a precipitate of elementalsulfur in situ which plugs the relatively highly permeable zones of theformation to obtain the improved vertical and/or horizontal sweepefficienty. If the pH of the relatively highly permeable zones is about6 or less, the initial treatment with the aqueous acid solution isunnecessary.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a method for improving the verticaland/or horizontal sweep efficiency of a subterranean oil-containingformation having a neutral to alkaline pH and having relatively highpermeability zones and relatively lower permeability zones. Theinvention is especially useful with secondary and tertiary recoveryprocesses wherein a displacing fluid is used to displace oil from asubterranean, oil-containing formation penetrated by at least oneinjection well and a spaced-apart production well in fluid communicationwith the formation. The displacing fluid can be any fluid whicheffectively displaces crude oil from the formation, e.g., it can be animmiscible, miscible or miscible-like displacing fluid. The displacingfluid is injected into the injection well to displace oil through theformation toward the production well from which oil is recovered.

In accordance with the present invention, first an aqueous weak orstrong acid solution is injected into the formation via the injectionwell which preferentially enters the relatively highly permeable zonesand lowers the pH of these zones to a pH of about 6 or less. Theconcentration of acid employed in the aqueous solution is determined bythe alkalinity of the relatively highly permeable zones of the formationto be plugged, particularly those zones in the vicinity of the injectionwell. For most purposes, concentration of the acid in the aqueoussolution is between about 5 and about 15 wt. % and can vary from smallconcentrations, e.g., 1 wt. %, up to saturation level within thesolution. Weak acids suitable for use in the present process include thewater soluble fatty acids such as formic, acetic, propionic and thelike, substituted water soluble organic acids such as chloracetic acid,hydroxyacetic acid and the like, various water soluble polycarboxylicacids such as citric acids and the like, or any cheap source of diluteacid. The preferred strong acid is hydrochloric acid. The quantity ofthe acid solution is that amount sufficient to extend into therelatively highly permeable zones of the formation a distance equal toat least 5 to 30 percent of the pore volume of the highly permeablezones between the injection well and the production well.

After the relatively highly permeable zones have been acid treated tothe desired pH, an aqueous solution of polysulfides, preferably ammoniumpolysulfide or ethylamine polysulfide, is injected into the formationvia the injection well that preferentially enters the relatively highlypermeable zones in the formation and allowed to remain in the highlypermeable zones of the formation for a sufficient period of time toundergo disproportionation under the pH conditions within the relativelyhighly permeable zones to form a precipitate of elemental sulfur in situwhich plugs the relatively high permeable zones. For example, thedisproportionate reaction for ammonium polysulfide under pH conditionsof about 6 or less would be as follows: ##STR1## The initial reaction(1) forms a sulfur precipitate and in reaction (2) hydrogen sulfidereacts with additional polysulfide to form a sulfur precipitate. Theconcentration of polysulfides in the aqueous solution is determined bythe amount of precipitational sulfur desired, and can vary from 20 wt. %to 70 wt. %. After the desired amount of plugging has been obtained,improved sweep efficiency is realized in displacing oil from theformation by subsequently injected fluids. Displacing fluids such aswater, gas or steam subsequently injected into the formation via theinjection well are diverted into less permeable zones containing oilthat is displaced through the formation toward the production well fromwhich oil is recovered.

Aqueous polysulfide solutions may be prepared by reacting elementalsulfur with an ammonium sulfide or ethylamine solution (20-70 wt.percent) or in concentrated ammonium hydroxide in the presence ofhydrogen sulfide. An example of the ammonium sulfide and sulfur reactionis as follows:

    (NH.sub.4).sub.2 S+3S→(NH.sub.4).sub.2 S.sub.4

The proportions of sulfide and sulfur are adjusted to fit the conditionsof the reservoir including porosity and permeability. Polysulfides mayalso be prepared in shear-thinning solutions of polysaccharides andpolyacrylamides for aid in selective injection. Even though conventionalpolymers would degrade under high temperature, the benefit ofselectively introducing the polysulfides in the formation would bejustified additional cost.

The volume amount of the aqueous polysulfide solution injected into theoil-containing formation is dependent upon formation characteristics andthe degree of plugging or partial plugging desired. Accordingly, theextent to which the permeability of the formation is reduced can becontrolled as desired. Preferably, the quantity of the aqueouspolysulfide solution required is that amount sufficient to extend intothe highly permeable zones a distance equal to at least 5 to 30% of thepore volume of the highly permeable zones between the injection well andthe production well. This amount will vary depending upon the porosity,thickness and oil and water saturation of the formation. Placement ofthe aqueous polysulfide solution into a particular strata of theformation may be accomplished by packing off the zone of interest bymechanical means such as packers, chokes and injection strings, etc., orby selective injection of chemicals due to natural differences in thereservoir rock.

In another embodiment of the invention, if the subterranean formationhas a natural pH of about 6 or less, the initial injection of theaqueous acid solution is unnecessary.

By the term "pore volume" as used herein is meant that volume of theportion of the formation underlying the well pattern employed asdescribed in greater detail in U.S. Pat. No. 3,927,716 to Burdyne et al,the disclosure of which is hereby incorporated by reference.

From the foregoing specification one skilled in the art can readilyascertain the essential features of this invention and without departingfrom the spirit and scope thereof can adapt it to various diverseapplications. It is my intention and desire that my invention be limitedonly by those restrictions or limitations as are contained in the claimsappended immediately hereinafter below.

I claim:
 1. A method for improving the vertical and/or horizontal sweepefficiency of a subterranean oil-containing formation having a neutralto alkaline pH and having relatively high permeability zones andrelatively low permeability zones, the formation being penetrated by atleast one injection well and a spaced-apart production well in fluidcommunication with the formation, the method comprising:(a) injecting anaqueous acid solution into the formation via the injection well thatpreferentially enters the relatively high permeability zones and lowersthe pH to about 6 or less; (b) injecting an aqueous polysulfide solutioninto the formation via the injection well that preferentially enters therelatively high permeability zones; and (c) allowing the aqueouspolysulfide solution in the relatively high permeability zones toundergo disproportionation under the pH conditions within the relativelyhigh permeability zones to form a precipitate of elemental sulfur insitu which plugs the relatively high permeability zones to obtain theimproved vertical and/or horizontal sweep efficiency.
 2. A methodaccording to claim 1 wherein the polysulfide is ethylamine polysulfide.3. A method according to claim 1 wherein the polysulfide is ammoniumpolysulfide.
 4. A method according to claim 1 wherein the amount ofaqueous acid solution and aqueous polysulfide solution injected into theformation is sufficient to extend into the relatively high permeabilityzones a distance equal to at least 5 to 30% of the pore volume of therelatively high permeability zones of the formation between theinjection well and the production well.
 5. A method according to claim 1wherein the acid is an inorganic acid, an organic acid, a water solublefatty acid, a substituted water soluble organic acid, or a water solublepolycarboxylic acid.
 6. A method according to claim 1 wherein the acidis acetic acid.
 7. A method according to claim 1 wherein the acid ishydrochloric acid.
 8. A method for improving the vertical and/orhorizontal sweep efficiency of a subterranean oil-containing formationhaving a pH of about 6 or less and having relatively high permeabilityzones and relatively low permeability zones, the formation beingpenetrated by at least one injection well and a spaced-apart productionwell in fluid communication with the formation, the methodcomprising:(a) injecting an aqueous polysulfide solution into theformation via the injection well that preferentially enters therelatively high permeability zones; and (b) allowing the aqueouspolysulfide solution in the relatively high permeability zones toundergo disporportionation under the pH conditions within the relativelyhigh permeability zones to form a precipitate of elemental sulfur insitu which plugs the relatively high permeability zones to obtain theimproved vertical and/or horizontal sweep efficiency.
 9. A methodaccording to claim 8 wherein the polysulfide is ethylamine polysulfide.10. A method according to claim 8 wherein the amount of aqueouspolysulfide solution injected into the formation is sufficient to extendinto the relatively high permeability zones a distance equal to at least5 to 30% of the pore volume of the relatively high permeability zones ofthe formation between the injection well and the production well.
 11. Amethod according to claim 8 wherein the polysulfide is ammoniumpolysulfide.